Recent advances in our understanding of cocaine addiction indicate that addiction involves mechanisms of neural plasticity. Immediate early genes (IEGs) are believed to play a role in mediating stimulus-induced neural plasticity and several laboratories have examined changes in gene expression that underlie the long-term neurochemical and behavioral laboratories have examined changes in gene expression that underlie the long-term neurochemical and behavioral effects of cocaine. Our laboratory has focused on the IEG termed Homer, which is rapidly induced by cocaine and appears to function at excitatory synapses to regulate the signaling coupling of metabotropic glutamate receptors (mGluR) and possibly NMDA receptors, to intracellular calcium stores. These glutamate receptors are enriched in neurons of the striatum and are known to be important in long-term neuronal plasticity, and in reward behaviors. Studies conducted during the prior funding period of this award have focused on the molecular and cellular functions of Homer. Homer possesses a N-terminal EVH1 domain that is essential for its interaction with mGluRs and other proteins. In Aim 1, we will determine the crystal structure of Homer EVH1 domain. We will also use yeast genetic approaches to provide detailed structure-function information regarding the specificity and regulation of Homer interactions. Aim 2 will examine the contribution of Homer to the function of metabotropic receptors in brain. A knock-in mouse will be generated that expresses a point mutant form of mGlur5 that does not interact with Homer. This genetic model will test the hypotheses that Homer is essential for normal mGluR signaling, for mGluR organization at the synapse, and for systems level plasticity that requires mGluRs; including cocaine sensitization. Aim 3 will continue our analysis of Homer-interacting proteins. The central hypothesis that is emerging from this work is that Homer functions to regulate the coupling of a specific set of membrane receptors to intracellular pools of releasable calcium. The contribution of Homer proteins to functional properties of ryanodine and inositol triphosphate receptors will also be examined. We will also characterize three novel proteins that have been identified to bind Homer. Preliminary studies indicate that these proteins may be important in coupling NMDA receptors to intracellular calcium pools, and in receptor trafficking. These studies promise to define important new mechanisms that contribute to the synaptic plasticity of cocaine addiction.